This invention relates to methods and apparatus for receiving a Digital Audio Broadcasting (DAB) signal, and more particularly, to such methods and apparatus that mitigate adjacent channel interference in the DAB signal.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM or FM channel allocation. IBOC systems promote economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested.
FM DAB systems have been the subject of several United States patents including U.S. Pat. Nos. 6,259,893; 6,178,317; 6,108,810; 5,949,796; 5,465,396; 5,315,583; 5,278,844 and 5,278,826. One FM IBOC DAB system uses a composite signal that includes orthogonal frequency division multiplexed (OFDM) subcarriers in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some IBOC options (e.g., the All-Digital option) permit subcarriers starting as close as 100 kHz away from the center frequency.
The digital portion of the DAB signal is subject to interference, for example, by first-adjacent FM signals or by host signals in Hybrid IBOC DAB systems. The FM Digital Audio Broadcasting signal is designed to tolerate interference in a number of ways. Most significantly, the digital information is transmitted on both lower and upper sidebands. The digital sidebands extend out to nearly 200 kHz from the center carrier frequency. Therefore an intermediate frequency (IF) filter in a typical FM receiver must have a flat bandwidth of at least ±400 kHz. One proposed First Adjacent Canceller (FAC) technique requires an approximately flat response out to about ±275 kHz from the center for effective suppression of a first adjacent signal. This would normally require an IF filter with a flat bandwidth of at least 550 kHz. A first adjacent cancellation technique is disclosed in U.S. Pat. No. 6,259,893, which is hereby incorporated by reference.
DAB systems utilize a specially designed forward error correction (FEC) code that spreads the digital information over both the upper and lower sidebands. The digital information can be retrieved from either sideband. However, if both sidebands are received, the codes from both the upper and lower sidebands can be combined to provide an improved output signal.
FM stations are geographically placed such that the nominal received power of an undesired adjacent channel is at least 6 dB below the desired station's power at the edge of its protected contour or coverage area. Then the D/U (desired to undesired power ratio in dB) is at least 6 dB. There are exceptions to this rule, however, and listeners expect coverage beyond the protected contour increasing the probability of higher interference levels.
At a station's edge of coverage, a second adjacent's nominal power can be significantly greater (e.g. 40 dB) than the host's nominal power within the desired coverage area. This can present a problem for the IF portion of the receiver where dynamic range is limited. The IF is where the IBOC DAB signal is converted from analog to digital. The sample rate and number of effective bits in the analog-to-digital (A/D) converter limit the dynamic range of the IF section.
A B-bit A/D converter has a theoretical instantaneous dynamic range of about (1.76+6*B) dB (maximum sinewave to noise ratio in its Nyquist bandwidth). For this discussion, assume that a practical AID converter has a dynamic range of 6 dB per bit of resolution. Oversampling of the signal of interest can improve the effective dynamic range by spreading the quantization noise over the larger Nyquist bandwidth of the A/D. The effect is to increase the dynamic range by one bit for each quadrupling of the sample rate. On the other hand, some headroom must be allowed in the A/D sampling to control clipping to an acceptable level.
As a practical IBOC DAB example, assume an 8-bit AID with 48 dB instantaneous dynamic range in its Nyquist bandwidth. Further assume a headroom of 12 dB peak-to-average ratio in the AGC, and another 10 dB of margin for fading and AGC “slop”. An oversampling ratio of 256 can increase the effective dynamic range in the signal bandwidth by 12 dB (in effect canceling the A/D headroom loss). Then the effective IF dynamic range in the IBOC signal bandwidth would be about 48 dB minus the 10 dB margin for fading, resulting in about 38 dB. If an instantaneous signal dynamic range of 28 dB in the signal bandwidth is required to detect the IBOC DAB signal without fading, then there is a margin of about 10 dB in the IF and A/D. This margin could be consumed by a large second adjacent signal entering the analog IF filter prior to A/D conversion.
It is a reasonable assumption that a good selective IF filter would suppress the second adjacent analog FM signal at 400 kHz away from FM center frequencies, but its IBOC sideband at 200 to 270 kHz from center would pass through the filter. If a second adjacent interferer is more than about +20 dB, then the dynamic range requirement of the A/D is increased by the excess second adjacent signal level above 20 dB. For example, if the second adjacent interferer is +50 dB, then the increased requirement above the minimum dynamic range is 30 dB, or about 5 more bits of A/D resolution above the minimum. However, there are ways to deal with the dynamic range issue other than the brute force method of increasing the bits in the A/D.
When a second adjacent interferer is +30 dB higher than the signal of interest, then the out-of-band emissions from it will likely corrupt the digital sideband on that side. Since corruption at that level will render that sideband useless, it may be preferable to filter out that sideband prior to A/D conversion. Filtering out the large second adjacent signal will restore the effective dynamic range eliminating the need for more bits of resolution. One way to approach this problem is to provide a set of selectable filters having different passbands for IF filtering prior to the A/D/converter.
Although the use of multiple filters may provide a good technical solution, the cost of the receiver is increased by the additional filters and switches. Also the accuracy of the filters may have an effect on cost.
There is a need for an improved method of minimizing the effects of first adjacent interference in IBOC DAB signals.